du: block size parsing

[BenWiederhake]

• -B0 shouldn't panic. I think this should win some kind of "panic golf" :)
• 1000 and kb and 1024 and k should result in different output, but parse_size_u64 causes information loss (i.e. does not return whether a multiplier was used).
• 0b11 should be accepted (and interpreted as 3), but determine_number_system does not recognize that prefix.

I think I'll fix this myself soon-ish, but wanted to documented it first, in case I forget about it.

[BenWiederhake]

More cursed invocations so that I don't forget:

BLOCK_SIZE=' 10' du --apparent-size README.md # 10
BLOCK_SIZE='10garbage' du --apparent-size README.md # 3G
BLOCK_SIZE='10MB' du --apparent-size README.md # 10MB
BLOCK_SIZE='10mB' du --apparent-size README.md # 10MB
BLOCK_SIZE='10mb' du --apparent-size README.md # 10MB
du --apparent-size -B 10KB README.md # 10KB
du --apparent-size -B 10kB README.md # 10KB
du --apparent-size -B 10MB README.md # 10MB
du --apparent-size -B 10mB README.md # 10MB
du --apparent-size -B 10mb README.md # error (sigh)
BLOCK_SIZE='10mib' du --apparent-size README.md # 10M
du --apparent-size -B 10mib README.md # error (sigh)
BLOCK_SIZE='10PiB' du --apparent-size README.md # 10P
BLOCK_SIZE='10piB' du --apparent-size README.md # 10 (lol)
du --apparent-size -B 10PiB README.md # 10P
du --apparent-size -B 10piB README.md # error
BLOCK_SIZE=' 10' du --apparent-size README.md # leading space should be trimmed
BLOCK_SIZE=$'\v10' du --apparent-size README.md # leading space should be trimmed, even though trim_ascii doesn't recognize it
BLOCK_SIZE=$'\xc2\xa03' du --apparent-size README.md # leading space should NOT be trimmed, even though trim_start would remove it
BLOCK_SIZE='3garbage' du --apparent-size README.md # 3g
BLOCK_SIZE='99999999TB' du --apparent-size README.md # Saturates!
dd if=/dev/zero of=/dev/null count=1 $'bs=\v10' # allows whitespace
dd if=/dev/zero of=/dev/null count=1 bs=0 # shouldn't crash (sigh)
dd if=/dev/zero of=/dev/null count=0b100 bs=1 # Doesn't support binary!