Use fixed strings for searching the password safe

Instead of passing the username as regular expression. This allows
matching usernames which contain regex syntax characters like
DOMAIN\user.

pwd.sh

@@ -87,7 +87,7 @@ read_pass () {
   get_pass "
   Enter password to unlock ${safe}: "
   printf "\n\n"
-  decrypt ${password} ${safe} | grep " ${username}" || fail "Decryption failed"
+  decrypt ${password} ${safe} | grep -F " ${username}" || fail "Decryption failed"
 }