MFC after:	1 week
Reported by:	markj
Fixes:		d34870708db9 ("install: Allow installing stdin")

(cherry picked from commit 0eef3d01c22a5e00cd2cf10c34871eddbfd22aa0)

This is mainy focused on using bool for booleans but also renames some
variables for clarity, adds some explicit comparisons, adds some braces,
with miscellanous style fixes thrown in.

MFC after:	1 week
Reviewed by:	imp
Differential Revision:	https://reviews.freebsd.org/D58355

(cherry picked from commit 134ddd372f8213885dd828ab38cec2384fd451db)

If from_name is "/dev/stdin" or "-" and the target is not a directory,
skip the comparison and copy data from standard input to the target.

MFC after:	1 week
Reviewed by:	imp
Differential Revision:	https://reviews.freebsd.org/D58348

(cherry picked from commit d34870708db9fa1eb8e29b5e085b755de1189b1f)

The five link flags get lost in prose.

Reviewed by:	ziaee
Sponsored by:	The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D57418

(cherry picked from commit 248dd56d2dea03e4723e8225b890d02fcc10973f)

Reported by:	des
Reviewed by:	ziaee
Sponsored by:	The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D57409

(cherry picked from commit b49021b49a78ca6a9e3a1a59c5aa6f2fec503afb)

Cannot strip the target if creating a link.

Reviewed by:	des
Sponsored by:	The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D57398

(cherry picked from commit 521afce6a859c1d7ac9674e8f21ff45418becaf5)

Reviewed by:	des
Sponsored by:	The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D57403

(cherry picked from commit 4908bea5b7f5de70032e201e718958ef40bc3b03)

Removes the file size limit for -C comparisons. The limit was
meant to prevent oversized mmap allocations, which is no longer
relevant as mmap is no longer used here (removed by
a0439a1b820fa0e742c00d095f5f5c06f5f19432, review D44809).
Credit to bdrewery.
See: https://reviews.freebsd.org/D57230

Reviewed by:	bdrewery, glebius, ziaee
Approved by:	glebius (mentor)
Obtained from:	Fudo Security
MFC after:	2 weeks
Sponsored by:	Fudo Security
Differential Revision:	https://reviews.freebsd.org/D57503

(cherry picked from commit 9d10b4d2c9e86d8f6ff1b654f468381a4a4cad6d)

Preserve metadata and prevent redundant disk writes during builds with
the install's -C (compare) flag.

The previous historical comparison limit of 16MB is insufficient for
modern toolchains, frequently choked or bypassed by a large base
components like LLVM/Clang, kernels, Rust apps, and large runtime
libraries.

By leaving matching files alone, install keeps their modification timestamps
intact. make(1) safely ignores those files on subsequent runs.

Base examples: 15.0 amd64 GENERIC kernel - 28MB, clang - 105MB, lldb - 97MB, etc.

Reviewed by:    glebius
Approved by:    glebius (mentor)
Obtained from:  Fudo Security
MFC after:      2 weeks
Sponsored by:   Fudo Security
Differential Revision:	https://reviews.freebsd.org/D57271

(cherry picked from commit 5a8e0e03ae86b449f29626b4f9db25d17c89b273)

Introduces the -z <max_cmp_size> flag, enabling users to set a custom file
size limit for pre-installation change checks and avoiding future hard-coded
limit modifications.

Reviewed by:    glebius
Approved by:    glebius (mentor)
Obtained from:  Fudo Security
MFC after:      2 weeks
Sponsored by:   Fudo Security
Differential Revision:	https://reviews.freebsd.org/D57230

(cherry picked from commit 97cad013a50a4012328e11424ed2350c1efc036c)