Currently, if `log4j2.isWebapp` is enabled, the effective values of the
following system properties:

* `log4j2.enableThreadlocals`,
* `log4j2.garbagefreeThreadLocalMap`,
* `log4j2.shutdownHookEnabled`.

is set to `false`.

After this PR:

* the above mentioned constants are set to the value **explicitly** set
  by the user,
* if the user didn't provide any value `!log4j2.isWebapp` is used.

Co-authored-by: Volkan Yazıcı <volkan@yazi.ci>

…webapp