Show nanoseconds in the -u/-c header line.
The present portability conditionals cannot handle the POSIX standard
st_mtim, so remove them and unconditionally use st_mtim.
PR: 218018
Reported by: jbeich
Reviewed by: bapt
Differential Revision: https://reviews.freebsd.org/D10145
Some of the modifications from the previous summer of code has been integrated
Modification for compatibility with GNU diff output has been added
Main difference with OpenBSD:
Implement multiple GNU diff options:
* --ignore-file-name-case
* --no-ignore-file-name-case
* --normal
* --tabsize
* --strip-trailing-cr
Make diff -p compatible with GNU diff
Implement diff -l
Make diff -r compatible with GNU diff
Capsicumize diffing 2 regular files
Add a simple test suite
Approved by: AsiaBSDcon devsummit
Obtained from: OpenBSD, GSoC
Relnotes: yes