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

Makefile

@@ -1,6 +1,4 @@
 BINARY  := chlog
-# chcat/chtail/chgrep are argv[0]-dispatched symlinks to the single chlog binary.
-LINKS   := chcat chtail chgrep
 DIST    := dist
 VERSION := $(shell git describe --tags --always --dirty 2>/dev/null || echo dev)
 GOFLAGS := -ldflags="-s -w -X main.version=$(VERSION)"
@@ -13,7 +11,6 @@ all: build
 
 build:
 	CGO_ENABLED=0 GOOS=$(OS) GOARCH=$(ARCH) go build $(GOFLAGS) -o $(DIST)/$(BINARY) .
-	@for l in $(LINKS); do ln -sf $(BINARY) $(DIST)/$$l; done
 
 test:
 	go test -v -race ./...
@@ -30,14 +27,12 @@ clean:
 install:
 	go install $(GOFLAGS) .
 
-# Generate bash/zsh/fish completions for chlog and each symlink entrypoint.
+# Generate bash/zsh/fish completions for chlog.
 completions: build
 	@mkdir -p $(DIST)/completions
-	@for b in $(BINARY) $(LINKS); do \
-		$(DIST)/$$b completion bash > $(DIST)/completions/$$b.bash; \
-		$(DIST)/$$b completion zsh  > $(DIST)/completions/_$$b; \
-		$(DIST)/$$b completion fish > $(DIST)/completions/$$b.fish; \
-	done
+	@$(DIST)/$(BINARY) completion bash > $(DIST)/completions/$(BINARY).bash
+	@$(DIST)/$(BINARY) completion zsh  > $(DIST)/completions/_$(BINARY)
+	@$(DIST)/$(BINARY) completion fish > $(DIST)/completions/$(BINARY).fish
 
 rpm: build rpm-package