argv: revert part of a3079e2

Since we have compat_strndup(), let's revert part of a3079e2 which used
malloc() + strncpy() instead of strndup(), in argv_remove_quotes().

In the meantime, constify arglen and unquotedlen.

Refs #286

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>