Fix arcstat.py handling of unsupported options

[loli10K]

Motivation and Context

Running arcstat.py with an unsupported option does not print the expected error and usage message:

root@linux:/usr/src/zfs# ./cmd/arcstat/arcstat.py -y
Traceback (most recent call last):
  File "./cmd/arcstat/arcstat.py", line 463, in <module>
    main()
  File "./cmd/arcstat/arcstat.py", line 436, in main
    init()
  File "./cmd/arcstat/arcstat.py", line 288, in init
    sys.stderr.write(msg)
TypeError: expected a character buffer object
root@linux:/usr/src/zfs# 

Description

This change allows the arcstat.py script to handle unsupported options gracefully and print both error and usage messages when one such option is provided:

root@linux:/usr/src/zfs# ./cmd/arcstat/arcstat.py -y
Error: option -y not recognized
Usage: arcstat.py [-hvx] [-f fields] [-o file] [-s string] [interval [count]]

	 -h : Print this help message
	 -v : List all possible field headers and definitions
	 -x : Print extended stats
	 -f : Specify specific fields to print (see -v)
	 -o : Redirect output to the specified file
	 -s : Override default field separator with custom character or string

Examples:
	arcstat.py -o /tmp/a.log 2 10
	arcstat.py -s "," -o /tmp/a.log 2 10
	arcstat.py -v
	arcstat.py -f time,hit%,dh%,ph%,mh% 1

root@linux:/usr/src/zfs# 

How Has This Been Tested?

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the ZFS on Linux code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• All new and existing tests passed.
• All commit messages are properly formatted and contain Signed-off-by.
• Change has been approved by a ZFS on Linux member.

[codecov]

Codecov Report

Merging #7799 into master will decrease coverage by 0.12%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #7799      +/-   ##
==========================================
- Coverage   78.32%   78.19%   -0.13%     
==========================================
  Files         373      373              
  Lines      112791   112791              
==========================================
- Hits        88341    88197     -144     
- Misses      24450    24594     +144

Flag	Coverage Δ
#kernel	78.47% <ø> (-0.06%)	⬇️
#user	66.85% <ø> (-0.67%)	⬇️

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