Fixes:	a99bf9966590859d21d28ec63798820dc07e0ba8

Historically, tftpd disallowed write requests to existing files
that are not publicly writable. Such requirement is questionable at least.
Let us make it possible to run tftpd in chrooted environment
keeping files non-world writable.

New option -S enables write requests to existing files
for chrooted run according to generic file permissions.
It is ignored unless tftpd runs chrooted.

Requested by:	marck
Differential:	https://reviews.freebsd.org/D41090 (based on)

(cherry picked from commit 273a307d0b80743fb08e23237b3f74dc94a8fa2a)
(cherry picked from commit 03c2616dc530e5b23f06f9aa421012154590e578)

uses gcc 4.2.1 for mips, powerpc and sparc64.

Revert "Revert part of r360964"

This reverts commit 2ebf10e7a1ced5d3dc4059d0595743277464d915.

Revert "Remove tests for obsolete compilers in the build system"

This reverts commit a606cb388f975561c37dbabc2fee82c27ef09929.

Assume gcc is at least 6.4, the oldest xtoolchain in the ports tree.
Assume clang is at least 6, which was in 11.2-RELEASE.  Drop conditions
for older compilers.

Reviewed by:	imp (earlier version), emaste, jhb
Sponsored by:	Dell EMC Isilon
Differential Revision:	https://reviews.freebsd.org/D24802

(cherry picked from commit fac6dee9eb58b2b558fec2aea749460ca623f6d6)

This reverts commit befc47822966, as it caused various CI build errors,
and we never merged the prerequisite commit fe815331bb40 ("build:
provide a default WARNS for all in-tree builds"). The latter is rather
intrusive, so I will avoid it.

Direct commit to stable/12 since the CI bots are broken.

Repeating the default WARNS here makes it slightly more difficult to
experiment with default WARNS changes, e.g. if we did something absolutely
bananas and introduced a WARNS=7 and wanted to try lifting the default to
that.

Drop most of them; there is one in the blake2 kernel module, but I suspect
it should be dropped -- the default WARNS in the rest of the build doesn't
currently apply to kernel modules, and I haven't put too much thought into
whether it makes sense to make it so.

(cherry picked from commit 7cc42f6d25ef2e19059d088fa7d4853fe9afefb5)

r358556 added alignas() use to the functional tests, which isn't defined
until C11. Raise the -std to C11 to fix the build under freebsd-gcc{6,9}.

Reported by:	mhorne, Jenkins/CI

(cherry picked from commit 924e10b809a9fcbc8688c1b5848f60b48e6103fe)

When dropping packets (RRQ or WRQ) for debugging, report the send
operation as successful. Reporting a failure stops the transfer
instead of using timeouts.

MFC r368521:
Fix the TFTP client when performing a RRQ for files smaller than 512 bytes
and the server not sending an OACK:
* Close the file.
* Report the correct the number of received blocks.

MFC r368647:
Improve the counting of blocks used to transfer a file from the
server to the client in case of not using an OACK: Don't miss
the first block in case of it is not also the last one.

MFC r368657:
When receiving a file having a length, which is a mulitple of the blocksize,
close the file once it is received.

358556:
Add support for the TFTP windowsize option described in RFC 7440.

The windowsize option permits multiple blocks to be transmitted
before the receiver sends an ACK improving throughput for larger
files.

360167:
Abort transfer if fseeko() fails.

tftpd: Check for errors from chdir()