libc: Fix constants used by `libc::pathconf`
libc: Fix constants used by `libc::pathconf`

`_PC_NAME_MAX` is necessary to use `libc::pathconf`. Its value is fixed
to 3 currently, but actually it varies with the platform.

* _PC_NAME_MAX == 3

Linux (glibc): https://sourceware.org/git/?p=glibc.git;a=blob;f=bits/confname.h;h=1c714dfbf9398b8a600f9b69426a7ad8c7e89ab4;hb=HEAD#l32
NaCl (newlib): https://chromium.googlesource.com/native_client/nacl-newlib/+/373135ec5241d09138aa56603742b94b3b64ea1d/newlib/libc/include/sys/unistd.h#430

* _PC_NAME_MAX == 4

Android (Bionic): https://github.com/android/platform_bionic/blob/7e919daeaad62515ebbbf7b06badc77625a14d90/libc/include/unistd.h#L59
FreeBSD: https://svnweb.freebsd.org/base/head/sys/sys/unistd.h?revision=239347&view=markup#l127
NetBSD: http://ftp.netbsd.org/pub/NetBSD/NetBSD-current/src/sys/sys/unistd.h
OS X: http://opensource.apple.com/source/xnu/xnu-2782.10.72/bsd/sys/unistd.h

This commit fixes this, and also addes the `_PC_PATH_MAX` constant
needed by further commits.

aidancully/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,GBGamer/rust,graydon/rust,graydon/rust,GBGamer/rust,aidancully/rust,graydon/rust,GBGamer/rust,aidancully/rust