unix: add various missing syscalls available on Linux

Various syscalls offered by x/sys/unix on Linux are not available on
Solaris and should be, such as Mkfifo, Getwd(), Futimes() and others.
In particular, all of the *at() variants of existing functions were
added where appropriate.

Getgroups() was fixed to use the correct value for its sanity check on
the maximum number of groups.

Utimesnano() was updated to use the native Solaris utimensat function
for setting nanosecond-precision time.

Utimes() was updated to have the same error semantics and checking as
other platforms.

Getgroups(), anysocktoaddr(), and Recvmsg() were fixed to check the
return value before assuming syscall failure instead of relying solely
on errno being set.

mksyscall_solaris.pl needed some updates to better match the output of
the one found in syscall.

mkerrors.sh needed some updates to work out of the box on Solaris,
matching those recently done to the one in syscall.

The signatures (names) of some function parameters were changed to be
consistent with other platforms for the sake of documentation.

Fixes #8609

Change-Id: I9e4e2fee6d3ecfad9f4d845a5702ffde5166e804
Reviewed-on: https://go-review.googlesource.com/14643
Reviewed-by: Aram Hăvărneanu <aram@mgk.ro>

unix/mkerrors.sh

@@ -12,12 +12,17 @@ export LC_ALL=C
 export LC_CTYPE=C
 
 if test -z "$GOARCH" -o -z "$GOOS"; then
-  echo 1>&2 "GOARCH or GOOS not defined in environment"
-  exit 1
+	echo 1>&2 "GOARCH or GOOS not defined in environment"
+	exit 1
 fi
 
 CC=${CC:-gcc}
 
+if [[ "$GOOS" -eq "solaris" ]]; then
+	# Assumes GNU versions of utilities in PATH.
+	export PATH=/usr/gnu/bin:$PATH
+fi
+
 uname=$(uname)
 
 includes_Darwin='
@@ -200,6 +205,7 @@ includes_OpenBSD='
 '
 
 includes_SunOS='
+#include <limits.h>
 #include <sys/types.h>
 #include <sys/socket.h>
 #include <sys/sockio.h>