Only sort the result array if it contains more than one element,

Only sort the result array if it contains more than one element,
making the mansearch() function easier to read for human auditors.
No functional change on OpenBSD.

As observed by Mark Millard <marklmi at yahoo dot com>, neither the
latest version of POSIX 2008 nor C11 defines what qsort(3) should do
for base == NULL && nmemb == 0.
My impression is it is indeed undefined behaviour because the
standards say that base shall point to an array, NULL does not point
to an array, and while there is special wording saying that compar()
shall not be called if nmemb == 0, i fail to see any similar wording
stating that base shall not be accessed if nmemb == 0.
Consequently, this patch is also likely to improve standard conformance
and portability.

Minor issue found by Stefan Esser <se at FreeBSD> with UBSAN.
He sent a patch to bugs@, but my patch differs in a minor way.