path: add Glob

path: add Glob

As discussed in http://groups.google.com/group/golang-dev/browse_thread/thread/926b7d550d98ec9e,
add a simple "path expander" function, which returns all the
files matching the given pattern. This function is called Glob
after glob(3) in libc.

Also add a convenience function, hasMeta, that checks whether
a string contains one of the characters which are specially handled
by Match.

R=rsc, r, r2
CC=golang-dev
https://golang.org/cl/2476041