Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Use libprocstat(3) to retrieve ELF auxiliary vector.

MFC after: 1 month

Handle AT_TIMEKEEP in procstat(1) -x [1]. Remove the AT_COUNT switch
case, since AT_COUNT is not an aux vector, it is the counter of total
number of defined vectors.

PR: bin/171662 [1]
MFC after: 1 week

Always return 0 if the sysctl failed.

This fixes the bug: when procstat -xa was run and the sysctl for a
process returned ESRCH or EPERM, for this process procstat output the
result collected for the previous successful process.

Make 64-bit procstat output ELF auxiliary vectors for 32-bit processes.

Reviewed by: kib
MFC after: 1 week

Don't output a warning if kern.proc.auxv sysctl has returned EPERM.
After r228288 this is rather a normal situation.

MFC after: 1 week

Cast a_val on printing. This fixes build on mips.

Make proctstat -x output more readable.

This also fixes the issue, spotted by mdf, with values that were
printed as decimal and had hex prefixes.

Discussed with: kib, rwatson
MFC after: 2 weeks

Fix build, hopefully.

Reviewed by: kib

Add new options, -e and -x, to display process environment variables
and ELF auxiliary vectors.

MFC after: 2 weeks