libbb: optionally honour libc provided SIGRTMIN/SIGRTMAX in get_signum()

libbb: optionally honour libc provided SIGRTMIN/SIGRTMAX in get_signum()

When an application documents that it responds such and such to
SIGRTMIN+n, that almost always means with respect to the libc-provided
SIGRTMIN. Hence I disagree with the "more correct" in commit
7b276fc17594. In any case, this is rather unfortunate:

36
34

(the first shell is bash). We probably can't change default behaviour
after 7 years, but at least we can provide a config option.

We avoid a little code generation (repeated calls to
__libc_current_sigrtmin) by stashing SIGRTMIN/SIGRTMAX in local
variables, but it does cost ~50 bytes. The next patch serves as penance
for that.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>