Some codepaths did not correctly parse configuration variables
specified with valueless "true", which has been corrected.
* jk/implicit-true:
fsck: handle NULL value when parsing message config
trailer: handle NULL value when parsing trailer-specific config
submodule: handle NULL value when parsing submodule.*.branch
help: handle NULL value for alias.* config
trace2: handle NULL values in tr2_sysenv config callback
setup: handle NULL value when parsing extensions
config: handle NULL value when parsing non-bools