Log: all: improve messages when parsing long integers

Previously, scan_ll() warned on errors, and some callers logged
additional messages, meaning that there could be up to 3 messages for
the same error. Also, the scan_ll() messages could be misleading in some
contexts.

Now, scan_ll() logs at debug level, and callers log errors when
appropriate. Also return pcmk_rc_bad_input ("Bad input value provided")
instead of EINVAL ("Invalid argument") when no number is found.