ignores stderr when reading PATH for the first time.

If .*rc has minor errors or warnings and stderr is used to prompt them, `shellenv/.firstline` gets the error message although PATH content is expected.
This commit discards stderr messages when issuing any shell command using `2> /dev/null`.
This method is supposed to work at least in POSIX OS and Cygwin environment (See: "POSIX devices" - http://cygwin.com/cygwin-ug-net/using-specialnames.html).
Signed-off-by: Sho Nakatani <lay.sakura@gmail.com>