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

Function is now standalone and is based on parse_all_tag. It should be faster too!

SIGPIPE has been reported on some embedded devices.

This is not duplicated code, sed expression are different in parse_tag() and parse_all_tag().
core.sh (captcha_process): fix regression concerning view method

Initial check: [ -z "$DISPLAY" ] for "view-x" and [ -c '/dev/fb' ] for "view-fb" were ingored (but debug message was correctly print).

Related commit:
core.sh: clarify --captchamethod
revision 90160fb8ac2485ed01899a1dd449608a6a22dfc4
upload.sh: fix --printf help message
*.sh: cosmetics: update header comment
docs (plowprobe.1): fix english typo
plowprobe: add message if provided url can be used with plowlist