stunnel: Update init script

stunnel: Update init script

The reworked init script:
* Loads and validates options using uci_validate_section() (through
  uci_load_validate())
* Allows service options be specified in the globals section
* Hard-codes less global options (debug, syslog), as their default
  values already work
* Adds support for almost all options (up to the current package
  version, 5.49)
* Moves the pid file into a subdirectory (/var/run/stunnel) so that it
  can be created successfully when setuid is used

Certain options are omitted:
* chroot - requires more setup than the init script can manage
* fips, libwrap - disabled at compile-time
* iconActive, iconError, iconIdle, taskbar - gui/win32 only
* verify - obsolete, verifyChain and/or verifyPeer should be used
  instead

Signed-off-by: Jeffery To <jeffery.to@gmail.com>