getusershell: Split file by lines instead of spaces.
* lib/getusershell.c: Include string.h and filename.h
(GNULIB_GETUSERSHELL_SINGLE_THREAD): Remove conditional to include
unlocked stdio functions that are no longer used.
(readname): Remove function.
(getusershell): Use getline and process the string instead of using
readname. Return the first absolute file name.
* modules/getusershell (Depends-on): Remove unlocked-io-internal.
Add getline and filename.
* doc/multithread.texi (Multithreading Optimizations): Don't mention
GNULIB_GETUSERSHELL_SINGLE_THREAD.