Update pv to 1.6.20

Summary:

• fix: add missing stddef.h include to number.c
• fix: starting pv in the background no longer immediately stops unless the transfer is to/from the terminal
• fix: using -B, -A, or -T now switches on -C implicitly
• fix: AIX build fixes
• i18n: updated German "--help" translations
• i18n: switched to UTF-8 encoding, added missing translations (de,fr,pt)
• docs: new "common switches" manual section
• docs: use placeholder instead of /dev/sda in the manual
• docs: correct shell in autoconf/scripts/index.sh
• cleanup: various compiler warnings cleaned up
• added common switches section to manual
• use placeholder instead of /dev/sda in the manual
• replace ash with sh in autoconf/scripts/index.sh
• fix AIX config check when the CWD contains "yes"
• updated German "--help" translations
• updated missing German translations and changed to UTF-8 encoding
• updated missing French translations and changed to UTF-8 encoding
• updated missing Portuguese translations, changed to UTF-8 encoding
• using -B, -A, or -T now switches on -C implicitly
• fixed numerous compiler warnings in newer GCC versions
• fixed bug where "pv /dev/zero >/dev/null &" stopped immediately
• marked unused arguments with GCC unused attribute, started using boolean data type for flags, corrected more compiler warnings

Test Plan:

• dmesg | pv -l -L 1

Reviewers: Triage Team

Differential Revision: https://dev.getsol.us/D12138

Format enhancement