Fix the following -Werror warnings from clang 3.5.0, while building
usr.sbin/rtadvd:

usr.sbin/rtadvd/rtadvd.c:1291:7: error: taking the absolute value of unsigned type 'unsigned int' has no effect [-Werror,-Wabsolute-value]
                    abs(preferred_time - pfx->pfx_pltimeexpire) > rai->rai_clockskew) {
                    ^
usr.sbin/rtadvd/rtadvd.c:1291:7: note: remove the call to 'abs' since unsigned values cannot be negative
                    abs(preferred_time - pfx->pfx_pltimeexpire) > rai->rai_clockskew) {
                    ^~~
usr.sbin/rtadvd/rtadvd.c:1324:7: error: taking the absolute value of unsigned type 'unsigned int' has no effect [-Werror,-Wabsolute-value]
                    abs(valid_time - pfx->pfx_vltimeexpire) > rai->rai_clockskew) {
                    ^
usr.sbin/rtadvd/rtadvd.c:1324:7: note: remove the call to 'abs' since unsigned values cannot be negative
                    abs(valid_time - pfx->pfx_vltimeexpire) > rai->rai_clockskew) {
                    ^~~
2 errors generated.

These warnings occur because both preferred_time and pfx_pltimeexpire
are uint32_t's, so the subtraction expression is also unsigned, and
calling abs() is a no-op.

However, the intention was to look at the absolute difference between
the two unsigned quantities.  Introduce a small static function to
clarify what we're doing, and call that instead.

Reviewed by:	hrs
MFC after:	3 days
Differential Revision: https://reviews.freebsd.org/D1197
sh: Prepend "$0: " to error messages if there is no command name.
Fix the following -Werror warnings from clang 3.5.0, while building
bsnmpd's snmp_hostres module:

usr.sbin/bsnmpd/modules/snmp_hostres/hostres_snmp.c:204:20: error: absolute value function 'abs' given an argument of type 'const long' but has parameter of type 'int' which may cause truncation of value [-Werror,-Wabsolute-value]
        str[9] = (u_char)(abs(tm->tm_gmtoff) / 3600);
                          ^
usr.sbin/bsnmpd/modules/snmp_hostres/hostres_snmp.c:204:20: note: use function 'labs' instead
        str[9] = (u_char)(abs(tm->tm_gmtoff) / 3600);
                          ^~~
                          labs
usr.sbin/bsnmpd/modules/snmp_hostres/hostres_snmp.c:205:22: error: absolute value function 'abs' given an argument of type 'const long' but has parameter of type 'int' which may cause truncation of value [-Werror,-Wabsolute-value]
        str[10] = (u_char)((abs(tm->tm_gmtoff) % 3600) / 60);
                            ^
usr.sbin/bsnmpd/modules/snmp_hostres/hostres_snmp.c:205:22: note: use function 'labs' instead
        str[10] = (u_char)((abs(tm->tm_gmtoff) % 3600) / 60);
                            ^~~
                            labs

Since tm::tm_gmtoff is a long, use labs(3) instead.

MFC after:	3 days
Change man(1) to use mandoc to render manpages

man(1) now first test the manpage to run with mandoc to make sure it can be
rendered.
In case groff cannot be found (because base has been built WITHOUT_GROFF) it
recommands to install groff from the packages
Fix typo
filedesc: plug a test for impossible condition in fgetvp_rights
Fix comments
Include required files directly instead of pollution via ufs/ufsmount.h.

Sponsored by:	Nginx, Inc.
buf.h is not needed here, and pollutes when ufsmount.h is included
from userland code.

Sponsored by:	Nginx, Inc.