Minor cleanup of EXCL_xxx_ERR macros

Delete EXCL_ACT_ERR and EXCL_SYS_ERR defines:
usage is removed in 43d2eeef5dd3a64c372dbf60e4a32aa9e97b4645:
    "dmesg: use err_exclusive_options()"
Mention CAP_SYSLOG in the dmesg manpage

CAP_SYSLOG is required for the --follow mode
if dmesg is launched with ordinary user rights.

CAP_SYS_ADMIN works too for now, but it's the deprecated case
(CAP_SYS_ADMIN on, CAP_SYSLOG off) and CAP_SYSLOG provides
more fine-grained control.
Add CAP_SYSLOG detection for dmesg

Now dmesg will check of CAP_SYSLOG if --follow is given
and no root privileges exists.
Depends on libcap-ng presence.