Refactor corridor styles into level styles

Corridor styles (any-to-any, circular/ring and star/hub) that were
determined in makecorridors() are now determined one call up in
makelevel().  This removes the need for last remaining inner function in
the code, as well as math.h and libm.  This also makes it easy to allow
level style to influence room placement in the future.

This is meant to go with commit 9a804348a8 (CS628: lev_comp: give
RANDOM_CORRIDORS optional arg) and commit 8206330d20 (improve circular
corridor algorithm).

sort_rooms_ring_comp() is based on code from K-Mod by Karadoc:

    https://github.com/karadoc/Nethack/commit/f43c48abc48c7058df81c513ac20faa3521e6ce1
Disable building server on Linux by default

Most people on Linux who'd want to build this probably aren't looking to
host a server; those who are can still enable it in the CMake
configuration screen.
Don't build netclient lib if it isn't used
Merge branch 'unnethack' of https://github.com/tung/NitroHack into unnethack
OS X builds with clang (LLVM) now, removed nested-functions flag and forced compiler in the docs