passwd: Cache the user/group tables

This is a significant optimization for conditions that need these
tables:

    Benchmark #1: ./bfs ~/code/linux -nouser >/dev/null
      Time (mean ± σ):     232.0 ms ±   2.5 ms    [User: 44.3 ms, System: 185.0 ms]
      Range (min … max):   228.7 ms … 238.7 ms    12 runs

    Benchmark #2: ./bfs-1.6 ~/code/linux -nouser >/dev/null
      Time (mean ± σ):      1.050 s ±  0.012 s    [User: 544.2 ms, System: 500.0 ms]
      Range (min … max):    1.025 s …  1.063 s    10 runs

    Benchmark #3: find ~/code/linux -nouser >/dev/null
      Time (mean ± σ):      1.040 s ±  0.012 s    [User: 533.6 ms, System: 500.6 ms]
      Range (min … max):    1.017 s …  1.054 s    10 runs

    Summary
      './bfs ~/code/linux -nouser >/dev/null' ran
        4.48 ± 0.07 times faster than 'find ~/code/linux -nouser >/dev/null'
        4.52 ± 0.07 times faster than './bfs-1.6 ~/code/linux -nouser >/dev/null'

1 files changed, 1 insertions, 0 deletions

diff --git a/Makefile b/Makefile
index 8121e2f..5bd8ab8 100644
--- a/Makefile
+++ b/Makefile
@@ -86,6 +86,7 @@ bfs: \
     mtab.o \
     opt.o \
     parse.o \
+    passwd.o \
     printf.o \
     spawn.o \
     stat.o \