Some consistency changes to library & headers flags.

Some consistency changes to library & headers flags.

Setting +nolib didn't disabled standard headers, causing unexpected
behaviour or forcing user to add +skip-std-headers. Also, setting any of
the POSIX libs ({posix,unix}[strict]) didn't enabled POSIX headers,
causing unexpected behaviour or forcing user to add +skip-posix-headers.
(This also is a code/documentation conflict, as the manual says posix
headers are skipped by default, which is false).

Modified behaviour by setting the standard and POSIX headers skipping in
relation to the usage of standard and POSIX libraries. This has the
side-effect that if a user specifies a header skipping flag prior to a
library flag, the header skipping flag will be discarded.

This behavioural change should be documented in the manual (but sadly is
not, for now).