df: Cleanup and refactor

[jlduran]

Summary

Improve code style and type safety:

• Change boolean flags from int to bool type for clarity
• Given sys/param.h is already included, use the MAX() macro it provides
• Update variable types from int to size_t
• Adjust string comparison logic for better readability
• Add --libxo to usage message
• Various style(9) fixes

No functional changes intended.

Suggested by: des

[jlduran]

• Remove stray reference to availheader

[des]

bin/df/df.c

59–66

285

305

465–467

481–483

586–589

601–606

611–614

otherwise it returns 0 for 0

[jlduran]

Address suggestions

[des]

bin/df/df.c

37–41

83–87

these should all be bool, you'll have to change makevfslist() and checkvfsname() to suit.

163–164

I'm not sure lflag is actually needed, we can just check whether vfslist_l is null like we do for vfslist_t.

294–296

this lets you drop the cast in the malloc() call below

308

309–310

this should be fatal, imo

368

should mntsize be size_t?

369–372

490–494

611–614

actually I was mistaken, I missed that the if above used <= instead of <... either revert this or change the if (and update the comment).

[jlduran]

Address suggestions.

bin/df/df.c

309–310

Will be addressed in a separate revision, as it is a functional change.

[jlduran]

bin/df/df.c

309–310

Addressed in https://reviews.freebsd.org/D55610

[jlduran]

• Add --libxo to usage message