Provide config:miscellany:enable-magic-file to control the use of
libmagic.  There are environment variables that do this already but
they are not specific to Subversion and affect other applications.

* subversion/include/svn_config.h
  (SVN_CONFIG_OPTION_ENABLE_MAGIC_FILE): New.

* subversion/include/private/svn_magic.h
  (svn_magic__init): Add config parameter.

* subversion/libsvn_subr/config_file.c
  (svn_config_ensure): Document new setting.

* subversion/libsvn_subr/magic.c
  (svn_magic__init): Check for enable-magic-file=no.

* subversion/libsvn_client/add.c
  (add): Pass config to svn_magic__init.

* subversion/libsvn_client/import.c
  (import): Pass config to svn_magic__init.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1572916 13f79535-47bb-0310-9956-ffa450edef68