fix(nfs): move fsid=N inside client parentheses in /etc/exports

exportfs parses 'path host(flags) fsid=N' as a separate fsid= token
without host or options, causing 'No options for path fsid=N' errors.
The correct format puts fsid=N inside the host parentheses:
  /path host(flags,fsid=N)

Changes:
- clientFlags(c, fsid) now appends fsid=N at the end
- buildExportFlags(e, fsid) same
- buildExportLine removed buildExportSuffix call; fsid is now per-client
- Removed now-unused buildExportSuffix function

Version: 0.5.3

Makefile

@@ -1,5 +1,5 @@
 BINARY=nasctl
-VERSION?=0.5.2
+VERSION?=0.5.3
 GO?=go
 LDFLAGS=-s -w -X github.com/darroyo/nasctl/internal/web.Version=$(VERSION) -X github.com/darroyo/nasctl/internal/web.Commit=$(shell git rev-parse --short HEAD 2>/dev/null || echo unknown)
 BUILD_FLAGS=CGO_ENABLED=0