patch: add dry-run alias for compatibility with other implementations.

Other implementations of patch(1), including GNU patch and "svn patch"
have a --dry-run option which does the same as our -C or --check
option.

Add a new alias to make our implementation more compatible.

Avoid zeroing during allocation.

This change reverts a change from OpenBSD which made use of
calloc, and therefore wasted time initializing arrays that
will later be realloc'ed.

Consistently use FreeBSD's reallocf():
- Drop some bogus casts to size_t.
- The new_p_foo variables are not needed anymore.

Also merge the changes from OpenBSD's manpage
patch.1 Rev 1.27:
"patch was moved from user portability (UP) to base in issue 7
and is no longer optional"

 - Remove $DragonFly$ tags as they are using git nowadays and VCS tags will
   not help merging.
 - Other changes to Copyright headers to make them consistent with other
   source code, we intend to fork from this point.

Reviewed by:	pfg

  DragonflyBSD and install it as bsdpatch.  WITH_BSD_PATCH makes it
  default and installs GNU patch as gnupatch.

Submitted by:	pfg
Obtained from:	The DragonflyBSD Project

pascal and vmstat.sparc.  All changed files on the vendor branch should
already have been imported.