miscellaneous fixes and improvements

[StefanKarpinski]

• Allowing the Tar.list callback to receive the raw header data (especially in the form of a vector of fields) is highly useful for analyzing wild tarballs. I implemented this when working on adding support for hard links so that I could look at a tarball containing hard links more easily.

• Adding a Header method that takes an existing Header object and modifies some of its fields is a pattern that's used in various places in the code base which this method makes cleaner and clearer.

• This makes all cases where the end of a tarball is encountered before it is expected throw the same EOFError whereas previously some places threw that error while others threw a generic error with a premature end of file message.

• This changes also makes the definition of a skip method for processes conditional so that when testing with a Julia version that has Tar as a stdlib (and thus already has this method defined), we don't break precompilation by redefining that method.

[codecov[bot]]

Codecov Report

Merging #104 (cffb931) into master (82cc7dd) will decrease coverage by 0.10%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #104      +/-   ##
==========================================
- Coverage   96.37%   96.26%   -0.11%     
==========================================
  Files           4        4              
  Lines         634      643       +9     
==========================================
+ Hits          611      619       +8     
- Misses         23       24       +1     

Impacted Files	Coverage Δ
src/Tar.jl	96.05% <ø> (-1.32%)	:arrow_down:
src/create.jl	97.05% <100.00%> (ø)
src/extract.jl	96.34% <100.00%> (+0.07%)	:arrow_up:
src/header.jl	94.20% <100.00%> (+0.17%)	:arrow_up:

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