Use a non-mmap backend for Plan9

[ghost]

This enables support for Plan9 by disabling the mmap backend on that platform. Plan9 uses 9P for all file-system interaction so mmap is impossible by design.

Tests pass on 9FRONT but sometimes fail due to a bug in sync.RWMutex.

The use case is eris-go which can serve a content-addressed file-system from a pogreb database, served via FUSE on Linux and 9P on Plan9.

Of all the Go databases in it's class, pogreb is the only one I've found that can actually work without mmap. And it's the fastest for my use case.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.24%. Comparing base (59b5287) to head (e4a8d66).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
fs/os_mmap.go	0.00%	2 Missing ⚠️
options.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #75      +/-   ##
==========================================
- Coverage   77.32%   77.24%   -0.09%     
==========================================
  Files          25       25              
  Lines        1804     1806       +2     
==========================================
  Hits         1395     1395              
- Misses        283      285       +2     
  Partials      126      126              

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[akrylysov]

Thank you for the contribution, exciting to see Pogreb working on Plan9!