Drop chcat/chtail/chgrep symlink entrypoints; ship chlog subcommands only

The RPM's /usr/bin/chcat conflicts with SELinux's
policycoreutils-python-utils on Fedora. Per Ben: remove the symlink
entrypoints entirely and ship only the chlog binary with cat/tail/grep
subcommands.

- Remove chcat/chtail/chgrep symlinks and their completions from
  nfpm.yaml, build-rpm.sh and the Makefile
- Remove the argv[0] dispatch in main.go; subcommands are unchanged
- Update the completion test to cover chlog only
- Update README usage to chlog cat|tail|grep

packaging/nfpm.yaml

@@ -24,17 +24,6 @@ contents:
       owner: root
       group: root
 
-  # chcat/chtail/chgrep dispatch on argv[0] inside the chlog binary.
-  - src: /usr/bin/chlog
-    dst: /usr/bin/chcat
-    type: symlink
-  - src: /usr/bin/chlog
-    dst: /usr/bin/chtail
-    type: symlink
-  - src: /usr/bin/chlog
-    dst: /usr/bin/chgrep
-    type: symlink
-
   # Shell completions (generated by scripts/build-rpm.sh before packaging).
   - src: dist/completions/chlog.bash
     dst: /usr/share/bash-completion/completions/chlog
@@ -48,39 +37,3 @@ contents:
     dst: /usr/share/fish/vendor_completions.d/chlog.fish
     file_info:
       mode: 0644
-  - src: dist/completions/chcat.bash
-    dst: /usr/share/bash-completion/completions/chcat
-    file_info:
-      mode: 0644
-  - src: dist/completions/_chcat
-    dst: /usr/share/zsh/site-functions/_chcat
-    file_info:
-      mode: 0644
-  - src: dist/completions/chcat.fish
-    dst: /usr/share/fish/vendor_completions.d/chcat.fish
-    file_info:
-      mode: 0644
-  - src: dist/completions/chtail.bash
-    dst: /usr/share/bash-completion/completions/chtail
-    file_info:
-      mode: 0644
-  - src: dist/completions/_chtail
-    dst: /usr/share/zsh/site-functions/_chtail
-    file_info:
-      mode: 0644
-  - src: dist/completions/chtail.fish
-    dst: /usr/share/fish/vendor_completions.d/chtail.fish
-    file_info:
-      mode: 0644
-  - src: dist/completions/chgrep.bash
-    dst: /usr/share/bash-completion/completions/chgrep
-    file_info:
-      mode: 0644
-  - src: dist/completions/_chgrep
-    dst: /usr/share/zsh/site-functions/_chgrep
-    file_info:
-      mode: 0644
-  - src: dist/completions/chgrep.fish
-    dst: /usr/share/fish/vendor_completions.d/chgrep.fish
-    file_info:
-      mode: 0644