core.sh: rewrite parse_attr() to avoid pipe "|head -n1"

Function is now standalone and is based on parse_all_attr.

SIGPIPE has been reported on some embedded devices.

See related issue 798:
| I managed to fix this problem.
| After some debugging I found that error occures at line 96 in putlocker.sh
|
| >> REL_PATH=$(parse_attr 'Download File' 'href' <<< "$PAGE") || return
|
| after removing "|| return" it works properly, but I dont't know why ? :)
core.sh (parse_all): drop 'a' useless label in sed expression

"b" is enough to branch to the end of the script.
Revert: core.sh (parse_all): drop 'a' useless label in sed expression

Regression introduced in revision cc8dc66b0fcbdfbf966210738fb038649599ab31
Label is explicitely needed.