Avoid executable headers in local include tree

Avoid executable headers in local include tree

sys_prototypes.h, afs_AdminErrors.h, and afs_Admin.h were being installed
into include/ in the build tree via ${INSTALL} instead of ${INSTALL_DATA}.
rx_prototypes.h was executable in the repository.  Install header files
non-executable and make rx_prototypes.h non-executable for cleanliness.

Discovered by Lintian because the executable permissions were duplicated
by make libafs_tree.

Change-Id: I609b62f69d1fecc2cdbba7e7442d93e9675c2fa0
Reviewed-on: http://gerrit.openafs.org/1129
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>