unsetenv_others and close_others only allows bool.

[ksss]

The execution option unsetenv_others and close_others only allows true or false.

$ ruby -e 'system("env", unsetenv_others: :ok)'
-e:1:in 'Kernel#system': expected true or false as unsetenv_others: :ok (ArgumentError)
	from -e:1:in '<main>'

$ ruby -e 'system("env", close_others: :ok)'
-e:1:in 'Kernel#system': expected true or false as close_others: :ok (ArgumentError)
	from -e:1:in '<main>'

Implementations:

• https://github.com/ruby/ruby/blob/53f1fc25462dceb93ffd9a481865e8dab5d5fb41/process.c#L2097-L2103
• https://github.com/ruby/ruby/blob/53f1fc25462dceb93ffd9a481865e8dab5d5fb41/process.c#L2121-L2127
• https://github.com/ruby/ruby/blob/53f1fc25462dceb93ffd9a481865e8dab5d5fb41/object.c#L3437-L3453