df: fix various compiler warnings and improve code quality

[aalx2176_gmail.com]

Summary

• Add __dead2 attribute to usage()
• Use correct types (ssize_t and size_t) for mount point counts and buffer lengths to avoid 64-to-32 truncation
• Fix invalid escape sequence in xo_emit format string
• Remove redundant sizeof(char) in malloc()

Test Plan

Compiled with -Weverything -Wno-unsafe-buffer-usage -Wno-sign-conversion and verified output manually