Cleanup sysconf(3) a bit. sysconf(3) assumes in some places it operates
on int, but in fact it should operate on long.

• Introduce 'lvalue' variable, which is long.
• Fix _SC_XOPEN_SHM for 64bit archs.
• Fix _SC_PHYS_PAGES for 64bit archs. Reported by: simokawa
• Use lvalue for pathconf(3), as it returns long.
• Cast value explicitly to long on return.