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