Add missing UDP multicast receive dtrace probes

Phabricator:	https://reviews.freebsd.org/D924
Reviewed by:	rpaulo markj
MFC after:	1 month
The value small=2**-(p+3), where p is the precision, can be determine from
lgamma(x) = -log(x) - log(1+x) + x*(1-g) + x**2*P(x) with g = 0.57...
being the Euler constant and P(x) a polynomial.  Substitution of small
into the RHS shows that the last 3 terms are negligible in comparison to
the leading term.  The choice of 3 may be conservative.

The value large=2**(p+3) is detemined from Stirling's approximation
lgamma(x) = x*(log(x)-1) - log(x)/2 + log(2*pi)/2 + P(1/x)/x
Again, substitution of large into the RHS reveals the last 3 terms
are negligible in comparison to the leading term.

Move the x=+-0 special case into the |x|<small block.

In the ld80 and ld128 implementaion, use fdlibm compatible comparisons
involving ix, lx, and llx.  This replaces several floating point
comparisons (some involving fabsl()) and also fixes the special cases
x=1 and x=2.

While here
  . Remove unnecessary parentheses.
  . Fix/improve comments due to the above changes.
  . Fix nearby whitespace.

* src/e_lgamma_r.c:
  . Sort declaration.
  . Remove unneeded explicit cast for type conversion.
  . Replace a double literal constant by an integer literal constant.

* src/e_lgammaf_r.c:
  . Sort declaration.

* ld128/e_lgammal_r.c:
  . Replace a long double literal constant by a double literal constant.

* ld80/e_lgammal_r.c:
  . Remove unused '#include float.h'
  . Replace a long double literal constant by a double literal constant.

Requested by:	bde
Update dl_iterate_phdr(3) to follow r272842

Relnotes:	yes
Sponsored by:	The FreeBSD Foundation
Build gperf only if we're using g++ (not clang++)

gperf is used as a build tool for g++ and is not needed for Clang
architectures. Ports and third-party software that need it can use the
up-to-date devel/gperf port.

PR:		194103 (exp-run)
Reviewed by:	bapt
Sponsored by:	The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D886
Fix a bug in decoding string indexes in snmp_target(3), thus causing
bsnmpd(1) to not send v3 notifications properly; while here add two
missing return statements which could lead to abort() in case of a
rollback
Restore the backed-out change, using __offsetof instead.
For gcc 4.6 and newer, _Static_assert is a keyword, so don't try to
redefine it. It does what we want, and is always available unlike
other alternatives.
accept(2) may and can return EAGAIN, document it.

MFC after:	1 week
Do not add late flag when file= is specified because it has a bad
side-effect.  The specified file should exist before the fstab line.

Reported by:	wblock (long time ago)
MFC after:	1 day